The vapor phase oxidation is known to be useful for producing, inter alia, acrylonitrile, acrylic acid, formaldehyde, maleic anhydride, phthalic anhydride, hydrogen cyanide, phenol and nitric acid. Conventionally, it is carried out with air in the presence of oxidation catalysts. Depending on the type of oxidizable reactants involved, reaction conditions may be varied to increase the desired product. Those reaction conditions are well known to one of ordinary skill in the art. The oxidation of ammonia, for example, is usually carried out at high temperature, with a short residence time, in order to maximize the formation of nitrogen oxide without the formation of undesired products, such as nitrogen. Through manipulating the temperature and pressure involved, the nitrogen oxide formed can be further oxidized to the desired nitrogen dioxide and/or to its dimer (NO.sub.2).sub.2, which are capable of being converted into nitric acid by absorption in water.
The above conventional oxidation process, however, is inefficient due to using a low amount of an oxidizable reactant and/or an excessive amount of nitrogen. A low concentration of the reactant, for example, is usually maintained in air due to the flammability limit of a reactant-air mixture. The use of this low concentration of the reactant, of course, reduces the yield of the desired product. Similarly, the presence of an excessive amount of nitrogen which is present in air reduces the production rate of the desired product since a large volume of nitrogen, which is not one of the reactants in the process, takes up much needed capacity or space in an oxidation process system.
To mitigate the shortcomings in this conventional oxidation process, the use of oxygen enriched air or free oxygen to oxidize the oxidizable reactant has been proposed, for example, in U.S. Pat. No. 3,927,182 - Powell. By increasing the concentration of oxygen in the oxidizing source, the quantity of nitrogen processed or introduced into an oxidation reactor system is substantially reduced. Such a reduction in nitrogen, of course, can increase the capacity of a given oxidation system since a greater amount of a reactant can be processed in an oxidation reactor in the absence of nitrogen gas. However, the application of oxygen enriched air or pure oxygen is limited or constrained in an oxidation system due to the flammable or explosive reactions associated with a high concentration of a reactant and/or oxygen in a reactant-oxygen mixture. Indeed, U.S. Pat. No. 3,927,182, in column 5, lines 25-40, teaches, for example, against using a high concentration of ammonia with an oxygen enriched air in nitric acid production systems while "Air best for formaldehyde and maleic" by Maux teaches against using pure oxygen, in lieu of air, in the production of formaldehyde and maleic. Such a constraint adversely affects the production of a large quantity of nitric acid, maleic, formaldehyde and other vapor phase oxidation products.
Accordingly, it is an objective of the present invention to increase the concentration of an oxidizable reactant in an oxidizable reactant-oxygen mixture in vapor phase reactions without incurring the risk of flammable or explosive reactions, thus increasing the production of the desired products.